A vacuum degassing apparatus has been conventionally known to remove bubbles generated in molten glass prior to forming of the molten glass in a forming apparatus, the molten glass being produced in a melting tank, so that the quality of glass products can be improved.
One structural example of such a vacuum degassing apparatus is shown in FIG. 7. The vacuum degassing apparatus 100 shown in FIG. 7 is an apparatus which is used in a process for vacuum-degassing molten glass G housed in a melting tank 101 and continuously supplying the vacuum-degassed molten glass to a subsequent treatment tank, and the interior of which is maintained in a reduced pressure state.
In the vacuum degassing apparatus 100, a vacuum vessel 105 is horizontally disposed in a vacuum housing 103, and the vacuum vessel has an uprising pipe 106 vertically mounted to a lower inlet side and a downfalling pipe 107 vertically mounted to a lower outlet side.
The uprising pipe 106 communicates with the vacuum vessel 105, and molten glass G prior to degassing treatment is sucked up from the melting tank 101 and is introduced into the vacuum vessel 105. The downfalling pipe 107 communicates with the vacuum vessel 105, and the degassed molten glass G is caused to come down from the vacuum vessel 105 and is discharged into the next treatment vessel (not shown). The vacuum housing 103 has a heat-insulating material 108, such as heat-insulating bricks) disposed therein so as to surround the vacuum vessel 105, the uprising pipe 106 and the downfalling pipe 107 for heat insulation covering.
The vacuum housing 103 is made of metal, such as stainless steel, and is vacuumed from outside by e.g. a vacuum pump (not shown) such that the interior of the vacuum vessel 105 housed in the vacuum housing is maintained in a certain reduced pressure, such as a reduced pressure state having about 1/20 to ⅓ atmospheric pressure for vacuum treatment.
In the vacuum degassing apparatus 100, since the molten glass G having a temperature, such as 1,200 to 1,400° C. is subjected to vacuum treatment, each of the vacuum vessel 105, the uprising pipe 106 and the downfalling pipe 107 needs to be made of a material having an excellent heat resistance and a low reactivity to the molten glass G having a high temperature.
When the vacuum degassing apparatus 100 has a small size, it is acceptable that each of the vacuum vessel 105, the uprising pipe 106 and the downfalling pipe 107 is made of a precious metal material having an excellent heat resistance, such as platinum or a platinum alloy. However, when each of the vacuum vessel 105, the uprising pipe 106 and the downfalling pipe 107 is made of a precious metal material in production equipment having a large size, the equipment cost is raised, which creates a problem of being difficult to realize such equipment.
From this point of view, as an apparatus to be capable of solving this problem, there is a proposed a vacuum degassing apparatus 120 shown in FIG. 8, which is configured such that a vacuum vessel 105, an uprising pipe 106 and a downfalling pipe 107 are made of refractory bricks, and the uprising pipe 106 and the downfalling pipe 107 have extension pipes 110 and 111 made of a precious metal material, such as platinum or a platinum alloy, connected to lower ends thereof (see Patent Document 1).